With an increasing rise in awareness of environmental problems, solar batteries as clean energy sources have attracted a great deal of attention. Various types of solar battery modules have been developed and proposed in recent years.
Generally, a solar battery module is fabricated, for example, by fabricating silicon solar cells or amorphous silicon solar cells, superposing a protective sheet, a filler layer, a layer provided with solar cells, i.e., photovoltaic cells, a filler layer and a back protective sheet in that order in a laminated structure, bringing those component layers into close contact by vacuum, and laminating those component layers by a lamination process.
The solar battery modules were applied to pocket calculators in the early stage of solar battery application and, subsequently, began to be applied to various kinds of electronic apparatuses. The field of civil application of solar battery modules have rapidly been spreading in recent years. The most important object of the solar battery module application will be the realization of large-scale, concentrated solar battery power generation.
The presently most common front surface protective sheet for the solar battery module is a glass sheet. Resin sheets, such as fluorocarbon resin sheets, have become notable in recent years and the development of resin sheets is in rapid progress.
Resin sheets excellent in strength are the most commonly used as the back protective sheet for the solar battery module. Metal sheets also are used as back protective sheets.
Generally, a protective sheet included in a solar battery module, for example, a front surface protective sheet, must be highly transparent to sunlight because the solar battery absorbs sunlight for photovoltaic power generation, and excellent in properties including strength, weather resistance, heat resistance, water resistance, light resistance, wind endurance, hailstorm resistance and chemical resistance. Particularly, the protective sheet must be excellent in moisture-proof property to prevent the permeation of moisture and oxygen, have a high surface hardness, excellent in soil resistance to prevent dust accumulation, have very high durability and protective ability. The back protective sheet must meet substantially the same conditions as those for the front surface protective sheet.
The glass sheet, which is the most commonly used at present as the front surface protective sheet of the solar battery module, has a high sunlight transmittance, is excellent in properties relating to durability, such as weather resistance, heat resistance, water resistance, light resistance and chemical resistance, is excellent in moistureproof property, has a high surface hardness, is excellent in soil resistance to prevent dust accumulation and has high protective ability. However, the glass sheet is inferior in strength, plasticity, impact resistance, workability, handling facility and cost.
Although the fluorocarbon resin sheet serving as the front surface protective sheet of the solar battery module, as compared with the glass sheet, is satisfactory in strength, plasticity, impact resistance and weight, the same is inferior in properties relating to durability including weather resistance, heat resistance, water resistance light resistance and chemical resistance, and is particularly unsatisfactory in moistureproof property and soil resistance.
Although a resin sheet having a high strength, as employed as the back protective sheet of the solar battery module, is satisfactory in strength, plasticity, impact resistance, weight and cost, the same is inferior in properties relating to durability including weather resistance, heat resistance, water resistance, light resistance and chemical resistance and, particularly, lacks moistureproof property and soil resistance.
There have been a proposal to use sheets of a material having an excellent gas-barrier property and impermeable to moisture, oxygen gas and the like as the front or the back surface protective sheet of the solar battery module. The most commonly used sheets having an excellent gas-barrier property are aluminum foils.
Although very excellent in gas-barrier property, aluminum foils cause problems in the disposal thereof and, basically, aluminum foils are opaque and obstruct the view of things behind them.
A resin film excellent in gas-barrier property, such as a film of a polyvinylidene chloride resin, or a polyvinyl alcohol resin or an ethylene-vinyl alcohol copolymer is a previously proposed resin film excellent in gas-barrier property. The resin film of a polyvinylidene chloride resin produces chlorinated gases when incinerated. Therefore, it is undesirable to use such a film in view of preventing environmental pollution. Basically, the gas-barrier property of that resin film is not necessarily satisfactory and that resin film is unsuitable for uses requiring a high gas-barrier property. The resin film of the polyvinyl alcohol resin or the ethylene-vinyl alcohol copolymer has a relatively excellent gas-barrier property in an absolute dry condition and is not satisfactory in impermeability to moisture. The impermeability of this resin film to oxygen gas deteriorates under a moist condition. Accordingly, this film is unsuitable, from the practical point of view, for use as a gas-barrier film.
A recently proposed gas-barrier film excellent in gas-barrier property is, for example, an inorganic oxide film, such as a silicon oxide film or an aluminum oxide film, deposited by a physical vapor deposition process, such as a vacuum evaporation process, or a chemical vapor deposition process, such as a low-temperature plasma chemical vapor deposition process.
The gas-barrier film formed by depositing such an inorganic oxide is an aggregate of inorganic oxide grains and inevitably has defects in its structure, which limits reliability in gas-barrier property. Since such a gas-barrier film has a glassy structure, the gas-barrier film is inferior in flexibility and is subject to cracking when mechanical stress is induced therein. When cracked, the gas-barrier property of the gas-barrier film deteriorates greatly.
A previously proposed composite gas-barrier film is a multilayer gas-barrier film comprising a plurality of component films formed by a multistage vapor deposition process. Another previously proposed composite gas-barrier film comprises a resin film having an excellent gas-barrier property and a deposited inorganic oxide film deposited on the surface of the resin film. These composite gas-barrier films are not necessarily satisfactory.